Pakistani security officials investigating the killing of Benazir Bhutto believe it was carried out by a single assassin with a gun and a bomb. Investigators have concluded that the man who fired at least three shots at her also detonated the suicide bomb that exploded a second later.

One of the principal pieces of physical evidence is his severed head. Plastic surgeons have reconstructed the face and printed the picture in two national newspapers, offering 10 million rupees (£78,000) for information leading to his identification.

A police officer has said he started running towards the gunman when he saw him raise his arm. 'He got there just too late, and he saw him lower his head just before the blast,' a senior Pakistani source said. 'That is what the suicide bombers have been told to do to obliterate their features.' In this case, however, the tactic appears to have failed.

A second man with a beard and shawl, captured in pictures of the assassination, turned out to be an innocent bystander.

The government is sticking to its assertion that the assassination was ordered by Baitullah Mehsud, an Islamic militant with al-Qaeda links from south Waziristan. US and British officials have said that an intercepted satellite telephone conversation in which Mehsud offers congratulations to a fellow militant for the killing appears credible.

Investigators have cross-checked thousands of calls made from the area around the assassination site in the run-up to the killing against lists of numbers used by suspected jihadis, leading to at least two arrests.

However, there are still conflicting accounts of Bhutto's cause of death, even within President Pervez Musharraf's government.

A Western official with extensive security expertise in Pakistan said he believed 'conclusively' that Bhutto was killed by the blast from the bomb and not by an assassin's bullet. 'I do know the shots didn't hit her. The Interior Ministry allowed me to be party to photos and other materials that showed it wasn't a bullet.'

He said all the evidence supported the Pakistani government's original - hotly contested - assertion that Bhutto died from a violent blow to the head caused by the suicide blast as she ducked into her vehicle. Film showing her headscarf lifting up after the shooting was caused by her attempt to quickly retreat into the car, he said.

However, a senior Pakistan government official said investigators were looking into the possibility that their initial conclusion was wrong and that she was shot.

The confusion served to emphasise the quagmire facing the five detectives from Scotland Yard's Counter-Terrorism Command wading into the middle of an inquiry that was mired in political controversy and severely botched long before they arrived. No autopsy was performed, the crime scene was hosed down a few hours after the 27 December assassination, and eye-witnesses left without being interviewed.

The official version of Bhutto's death, that she hit her head on the lever of her car's sunroof, drew howls of derision from the opposition, but a Musharraf aide said it was based on the available medical evidence. 'The doctors said the only wound was on the right side of the head. The left side was intact and the shots and blast were from the left,' he said.

Now officials are considering the possibility that Bhutto turned her head after she heard the first shot and was then hit on the right side by the second or third shots.

Another possibility is that the doctors missed an entry wound on Bhutto's left side. 'The doctor [Mohammad Khan] was asked, 'Did you examine the other side of her head?' He said, 'I didn't have time. They all thought there would be an autopsy,' the government official said.

Bhutto's widower and successor as leader of the Pakistan People's Party (PPP), Asif Ali Zardari, refused to allow an autopsy.

There were no pathologists among the seven doctors who signed the medical report and the hospital was mobbed with Bhutto supporters, who even pushed their way into the operating theatre.

For all the evidence of police ineptitude, there is no evidence of a government conspiracy. Bhutto would almost certainly be alive if she had not ignored police advice and stopped her armoured car on her way from a political rally, so that she could stand up in her socks on the rear seat and poke her head out of the sunroof to wave at supporters.

'She made herself such an easy target that a 16-year-old boy could have walked up and got her,' a Western security expert said.